1. Field of the Invention
The present invention relates to a magnetooptical reproduction apparatus and method for reproducing information recorded in a magnetooptical recording medium and, in particular, to a magnetooptical reproduction apparatus and method for reproducing recorded information using a domain wall displacement detection technique.
2. Description of the Related Art
The linear recording density of an optical disk is largely dependent on the laser wavelength of a reproducing optical system used to reproduce/pick-up information recorded on a magnetooptical recording medium, and the numerical aperture of an objective lens used in the optical system. A beam waist diameter 2Wo is determined by the laser wavelength xcex of the reproducing optical system and the numerical aperture NA of the objective lens; thus, the relationship of 2Wo=Kxc2x7xcex/NA holds, where xcex/NA and K are constants determined by the aperture of the lens and the intensity distribution of an incident luminous flux. Spatial frequency during a signal detection is subject to a limitation of 2NA/xcex.
Optical disks having a higher storage capacity are highly desired. Efforts to increase storage capacity and improve information reproduction by improving control of the laser wavelength xcex and the numerical aperture NA face a limitation in an attempt to achieve a high-density recording in optical disks in excess of the diffraction limit determined by the wavelength xcex and the numerical aperture NA of the conventional optical disk. Conventional techniques for improving the recording density of the optical disk have been developed by improving the structure of the recording medium and the method of reading out information from the recording medium.
For example, Japanese Patent Laid-Open No. 3-93058 and Japanese Patent Laid-Open No. 6-124500 disclose a signal reproduction method. In this disclosure, a signal is recorded on a multilayer recording medium formed of magnetically coupled reproducing and recording layers. The reproducing layer is irradiated with a laser beam that raises the temperature in a localized area of the laser beam after aligning the magnetization direction of the reproducing layer. An information (data) signal digitally recorded as a series of magnetic domains in the recording layer serially is transferred to the localized area having a raised temperature in the reproducing layer and is read out at the same time. This method limits, to a tiny area relative to a reproducing laser spot diameter, an aperture which is heated by the laser beam so at to reach a transfer temperature sufficient for a signal to be detected therefrom. This disclosed technique reduces inter-code interference during reproduction, and reproduces a signal having a bit period shorter than the optical detection limit xcex/2NA.
This type of reproduction method is called an MSR (magnetically induced superresolution) readout method. Since the MSR reproduction method has an effective signal detection area smaller than the reproducing laser spot diameter, the amplitude of a reproduction signal disadvantageously gets smaller.
Japanese Patent Laid-Open No. 6-290496 discloses another reproduction method. In this disclosure, a domain wall present in the border of a magnetic domain (recording mark) is displaced toward a higher-temperature direction due to a wall energy gradient, and a high-density recorded signal is reproduced by detecting the displacement of the domain wall. In this method, the domain wall moves at the moment the domain wall is heated to a temperature at which the domain wall becomes movable. The high-density recorded signal is reproduced by detecting the displacement of the domain wall. The signal reproduction is possible without regard to the optical diffraction limit. This type of reproduction method is called a DWDD (domain wall displacement detection) method.
Theoretically, the DWDD method is independent of the length of a recording mark. The recording medium itself moves with respect to a reproducing light beam spot and the speed of the domain wall displacement is definite. A linear velocity or a mark length, practically reproducible, is subject to a limitation. Specifically, when a signal recorded on DWDD medium is reproduced using a conventional optical system (for example, under the conditions of xcex=680 nm, and NA=0.55), the rotational speed of the medium must be slowed to 2 m/s or lower to perform a DWDD reproduction which stably reproduces a signal with a miniature recording mark having a mark length of 0.1 xcexcm or shorter. The conventional DWDD reproduction method cannot speed up the data transfer rate.
Accordingly, it is an object of the present invention to provide a magnetooptical reproducing apparatus that implements an improved DWDD reproduction method, achieving a fast data transfer rate.
In one aspect, the present invention relates to a magnetooptical reproducing apparatus that reproduces information recorded on a magnetooptical recording medium by moving a domain wall of a recording domain of the magnetooptical recording medium in accordance with a temperature distribution formed in the magnetooptical recording medium. The magnetooptical reproducing apparatus includes an optical system that directs an optical beam to the recording medium to form a localized temperature distribution in the magnetooptical recording medium, a drive mechanism that moves the optical beam and the recording medium relative to one another, and a reproducing circuit that generates a reproduction signal in response to detected variation in the optical beam reflected from the recording medium, wherein the optical system satisfies a condition of 2Wo less than 900 nm, where 2Wo is a beam waist diameter of the optical beam.
In another aspect, the present invention relates to a magnetooptical reproducing method for reproducing information recorded on a magnetooptical recording medium by detecting movement of a domain wall of a recording domain of the manetooptical recording medium caused to move in accordance with a temperature distribution formed in the magnetooptical recording medium. The magnetooptical reproducing method includes a step of directing an optical beam to the magnetooptical recording medium to form the temperature distribution in the magnetooptical recording medium, where the optical beam has a beam waist diameter of 2Wo less than 900 nm, a step of moving the optical beam and the recording medium relative to one another, a step of detecting an optical beam reflected from the recording medium, and a step of forming a reproduction signal in response to the result of the detecting step according to data recorded on the magnetooptical recording medium.
Further objects, features, and advantages of the present invention will be apparent from the following description of the preferred embodiments with reference to the attached drawings.